1. Field of the Invention
The present invention relates to a developer holding member, a method of producing a developer holding member and an image-forming apparatus and in particular to a developer holding member used in a copier or printer utilizing electrophotographic method, a method of producing a developer holding member, a developing apparatus and an image-forming apparatus.
2. Description of the Related Art
In a copier or printer utilizing an electrophotographic process, an electrostatic latent image formed on a photoreceptor is developed by electrostatically adsorbing a developer onto the electrostatic latent image, and a cylindrical developer holding member is used to supply a developer. In such development, an amount of developer depending on the charging potential of a photoreceptor must be supplied to the electrostatic latent image.
However, when a developer of small particle diameter or a developer of high charging performance is used, a development performance distribution of the developer on the developer holding member is generated depending on the development history, and as a result, in some cases developer of an amount according to the charging potential of the photoreceptor is not supplied. The cause of this phenomenon, called ghost development, can be qualitatively explained below.
FIG. 5 shows an outline of a developing apparatus using a magnetic toner. The developing apparatus is composed of a developer holding member 11, a magnet 12, a developer hopper 13 and a developing blade 14. A developer 15 is stored in the developing hopper 13, and the developer is attracted towards the developer holding member 11 by the magnetism of the magnet 12. By rotating the developer holding member 11, the developer, having adhered to the developer holding member, is regulated by the developing blade 14 so as to have a predetermined thickness. The developer is electrostatically charged by friction among developer particles and friction between the developer and the developing blade 14. In a position near to an electrostatic latent image holding member 16, the charged developer is transferred by Coulomb force onto an electrostatic latent image on the electrostatic latent image holding member thereby visualizing the electrostatic latent image. Among developer particles on the developer holding member 11, only the developer positioned on a part corresponding to the electrostatic latent image is consumed in visualization of the latent image. By rotation of the developer holding member, new developer is supplied to the consumed portion and charged by the developing blade 14.
Because the development is carried out in this way, new developer supplied to the portion where the previous developer was consumed in the developing process undergoes frictional charging only once by the developing blade, whereas the developer on the portion where it was not consumed in the process undergoes frictional charging again.
As a result, the charging amount of the developer on the developer holding member 11 becomes distributed depending on the development history. As the charging amount is increased, the Coulomb interaction between the developer and the electrostatic latent image is increased and simultaneously the attraction between the developer and the developer holding member is also enhanced due to the image force. The amount of developer transferred onto the electrostatic latent image, that is, development performance, is determined by the relationship in magnitude between these forces.
In actual development, therefore, there occur cases where the development performance of the portion newly supplied with developer becomes higher or lower than that of other portions, accordingly an image different from the electrostatic latent image appears in the resulting print.
For example, consider copying a manuscript having a written portion “AAAAA” and a halftone region of uniform density, as shown in FIG. 6A. Usually, the circumferential speed of the developer holding member 11 is higher than that of the electrostatic latent image holding member 16, however for the sake of description, it is assumed that the two have the same circumferential speed. It is also assumed that development proceeds downward in the FIG. 5.
Because the perimeter of the developer holding member is generally shorter than the length of the manuscript, the developer holding member has to be rotated several times for copying one manuscript. In FIGS. 6B and 6C, it is assumed that the length “L” is the perimeter length of the developer holding member. By developing this portion, a developer layer, in which development performance is distributed according to the electrostatic latent image, is formed on the surface of the developer holding member, and this layer is used in development of a next portion. In this case, if the development performance of the developer used in developing the letters is higher than that of the developer on the other portion, an image called a positive ghost, absent in the electrostatic latent image, can appear on the position corresponding to the perimeter (length L) of the developer holding member as a result of development, as shown in FIG. 6B. However, if the development performance in that portion is lower, there occurs a phenomenon called a negative ghost, as shown in FIG. 6C, in which although the electrostatic latent image is present, the electrostatic latent image is not developed.
As described above, the ghost development is related to the charging performance of the developer and thus becomes particularly significant where developer of small particle diameter or developer of improved charging performance is used.
For example, a method of suppressing generation of the ghost development by providing the surface of a developer holding member with a phenol resin- and carbon-containing resin layer having electrical conductivity and surface lubricating properties is disclosed as a technique for suppressing generation of ghost development (see for example, Japanese Patent Application Laid-Open (JP-A) No. 2000-231257).
Also, a method of suppressing generation of ghost development by providing the surface of a developer holding member with a coating of molybdenum is also disclosed (see, for example, JP-A No. 7-281517).
In the method which includes providing the surface of a developer holding member with a resin layer, however, the resin layer is abraded easily thus changing the surface profile of the sleeve with time during running, giving rise to the problem that there is only a short period of time during which the ghost development can be suppressed to a certain degree.
In the method which includes providing the surface of a developer holding member with the molybdenum layer, the charging properties of molybdenum are low where recently developed toners for fixing at low temperatures are used, thus giving rise to the problem of reduction in the density of the resulting image.